1. Field of the Invention
The present invention relates in general to an electronic ballast for discharge lamps such as fluorescent lamps, and more particularly to an electronic ballast circuit for discharge lamps which is capable of generating a high voltage required to light the discharge lamps using a transformer and limiting amounts of currents flowing to the discharge lamps after the lighting of the discharge lamps, so that an amount of light from the discharge lamps can be maintained constantly.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a circuit diagram of a conventional electronic ballast circuit for discharge lamps. As shown in this drawing, the conventional electronic ballast circuit comprises transistors Q1 and Q2 and a transformer T for driving bases of the transistors Q1 and Q2. The transistor Q1 has a collector connected to a power source terminal Vcc, which is also connected to a condenser C1 through a resistor R1. A connection point of the resistor R1 and the condenser C1 is connected to a secondary winding T22 of the transistor base driving transformer T through a diac DA1 and to a base of the transistor Q2 through parallel connected condenser C3 and resistor R3. The connection point of the resistor R1 and the condenser C1 is also connected commonly to an emitter of the transistor Q1 and a collector of the transistor Q2 through a diode D1. The connection point of the resistor R1 and the condenser C1 is also connected to a base of the transistor Qt through another secondary winding T21 of the transformer T and parallel connected condenser C2 and resistor R2. A common connection point of the emitter of the transistor Q1 and the collector of the transistor Q2 is connected to filament terminals of discharge lamps LP1 and LP2 through a primary winding T11 of the transformer T and reactors L1 and L2. The power source terminal Vcc is also connected commonly to the other filament terminals of the discharge lamps LP1 and LP2 and a condenser C5 through a condenser C4. A condenser C6 is connected between the filament terminals of the discharge lamp LP1 and a condenser C7 is connected between the filament terminals of the discharge lamp LP2. The reference numerals D2 and D3, not described, designate diodes for protecting the transistors Q1 and Q2 when the transistors Q1 and Q2 are turned on/off, respectively.
The operation of the conventional electronic ballast circuit with the above-mentioned construction will hereinafter be described.
Upon application of an electric power to the power source terminal, the power is applied to the second filament terminals of the discharge lamps LP1 and LP2 and the condenser C5 through the condenser C4. The power is also applied to the condenser C1 through the resistor R1. As a result, the condenser C1 charges. The power is also applied to the first filament terminals of the discharge lamps LP1 and LP2 through the diode D1, the primary winding T11 of the transformer T and the reactors L1 and L2. As a result, the condensers C6 and C7 charge.
When a charging voltage on the condenser C1 reaches a level for biasing the diac DA1 into conduction, the diac DA1 conducts, thereby causing the charging voltage on the condenser C1 to be applied to the base of the transistor Q2 through the diac DA1 and the parallel connected condenser C3 and resistor R3. As a result, the transistor Q2 is turned on.
When the transistor Q2 is turned on, a current from the condenser C4 flows through the second filament terminals of the discharge lamps LP1 and LP2, the condensers C6 and C7, the first filament terminals of the discharge lamps LP1 and LP2, the reactors L1 and L2, the primary winding T11 of the transformer T and the transistor Q2.
The condensers C6 and C7 and the reactors L1 and L2 constitute resonance circuits, respectively. A quality factor Q of each resonance circuit is as follows: EQU Q=.omega.L/R=1/.omega.CR
At an initial condition, since a resistance across each of the discharge lamps LP1 and LP2 is infinite, a voltage of several hundred volts is induced across each of the discharge amps LP1 and LP2 in proportion to the Q value, resulting in an initial discharge thereof.
Thereafter, when no current from the reactors L1 and L2 flows toward the primary winding T11 of the transformer T at a resonance frequency (f=1/2.pi..sqroot.LC) because of the resonances by the condensers C6 and C7 and the reactors L1 and L2, a high voltage is induced at each dotted point of the secondary windings T21 and T22 of the transformer T, thereby causing a low voltage to be applied to the base of the transistor Q2 and a high voltage to be applied to the base of the transistor Q1. As a result, the transistor Q2 is turned off and the transistor Q1 is turned on.
In the case where the transistor Q2 is turned off and the transistor Q1 is turned on in the above manner, a current from the transistor Q1 flows to the condenser C5 through the primary winding T11 of the transformer T, the reactors L1 and L2 and the discharge lamps LP1 and LP2.
Then, when no current from the primary winding T11 of the transformer T flows toward the reactors L1 and L2 because of the resonances by the reactors L1 and L2 and the condensers C6 and C7, a counter electromotive force is generated in the primary winding T11 of the transformer T, with its polarity being opposite to that in the previous case. The counter electromotive force in the primary winding T11 of the transformer T causes a low voltage to be induced at each dotted point of the secondary windings T21 and T22 of the transformer T, thereby causing a high voltage to be applied to the base of the transistor Q2 and a low voltage to be applied to the base of the transistor Q1. As a result, the transistor Q2 is turned on and the transistor Q1 is turned off.
In this manner, the transistors Q1 and Q2 are repeatedly turned on/off because of the resonances by the reactors L1 and L2 and the condensers C6 and C7, so that the high voltage can be generated across each of the discharge lamps LP1 and LP2. The high voltages allow the discharge lamps LP1 and LP2 to discharge and, thus, to generate light.
However, the conventional electronic ballast circuit has a disadvantage, in that the discharge of the discharge lamps is performed by applying the high voltage directly to the filaments thereof, resulting in a blackening occurring at a discharging start point of time. The blackening shortens the life of the discharge lamps. Also, the reactors and condensers are required to constitute the resonance circuits in proportion to the number of the discharge lamps. For this reason, particularly when a single ballast circuit is used for a plurality of (at least two) discharge lamps, it is impossible to make a size of the ballast circuit small and a weight thereof light and the cost is increased, due to an increase in the number of the components.